In a Triple Ko, there are three Kos on the board. all in play at the same time. Often they are part of the same configuration, as in this example.
Say you have the kos a, b and c. You take a, your opponent takes b, you take c. Your opponent retakes a, you retake b and he retakes c. With the simple ko rule, you could now retake a again, and the cycle would go on for ever.
Of course, this is undesirable. Traditionally, a triple ko meant no result, and was considered unlucky.
In October 1998, the fourth game of the Meijin title match
ended without result when neither O Rissei nor Cho Chikun would yield in a triple ko.
Several modern rulesets handle it with a superko rule.
See the /Discussion page for more.